Recently, due to the sudden increase in oil prices, research has been actively conducted on hybrid-type construction machinery having improved fuel efficiency by storing surplus power from an engine in a battery and by supplying power from a battery to make up for insufficient power from an engine.
Such a system that uses both an engine and an electric motor as a source of power and has an electric energy storage device is called a hybrid system. For example, hybrid systems include hybrid systems for hybrid vehicles and heavy machinery such as excavators.
A typical excavator system, uses an engine as a source of power to perform an operation of rotating or propelling a boom, an arm, and a bucket that embody the final load, by means of a medium that is hydraulic pressure. Conversely, a hybrid excavator system additionally includes 2 motors and electricity storage devices installed on a typical excavator, so as to improve the overall efficiency of the excavator system. Main components that are added to a hybrid excavator system include a motor, an electricity storage device, an inverter, and a converter. Here, the electricity storage device includes a battery and an ultra-capacitor.
FIG. 1 is a configurative diagram of a power converting device including a DC-DC converter according to the related art.
Referring to FIG. 1, a power converting device 100 including a DC-DC converter according to the related art includes a switched mode power supply unit 110, a logic control board 120, an engine auxiliary motor inverter 130, a load motor inverter 140, a DC link capacitor 150, and an ultra-capacitor converter 160 that is a DC-DC converter. Here, the switched mode power supply unit 110, the logic control board 120, the engine auxiliary motor inverter 130, the load motor inverter 140, and the ultra-capacitor converter 160 are connected to a battery 101 for a control board, an excavator electric device 102, an engine auxiliary motor 103, a load motor 104, and an ultra-capacitor 105, respectively.
The switched mode power supply (SMPS) 110 is connected to the battery 101 for a control board and supplies power to the logic control board 120.
The logic control board 120 senses the power of the ultra-capacitor 105 and the power of the DC link capacitor, and performs the function of controlling an initial driving logic.
The engine auxiliary motor inverter 130 performs the function of charging the DC link capacitor 150 through the engine auxiliary motor 103. Here, the engine auxiliary motor 103 is directly connected to an engine, and rotates at the same rpm as the engine during engine operation.
When a power contactor of the ultra-capacitor 105 is turned ON, the load motor inverter 140 performs the function of driving the load motor 104 according to a charged voltage. Here, the load motor 104 supplies the power required by driven components that may be used for electric power, from among the driven components for the tasks of the excavator.
The DC link capacitor 150 charges a DC voltage converted by the engine auxiliary motor inverter 130. The DC link capacitor 150 is connected to the ultra-capacitor converter 160.
The ultra-capacitor converter 160 uses the electric energy stored in the DC link capacitor 150 to perform the function of charging the ultra-capacitor 105. The ultra-capacitor converter 160 is connected between the DC link capacitor 150 and the ultra-capacitor 105. Here, the ultra-capacitor 105 is charged by a voltage converted by the ultra-capacitor converter 160.
The power converting device 100 including a DC-DC converter configured as above includes an inverter (for example, the engine auxiliary motor inverter 130 and the load motor inverter 140) for driving a motor, and a converter (for example, the ultra-capacitor converter 160) for driving an ultra-capacitor. Here, the process of converting a voltage of the DC link to charge the ultra-capacitor 105 by the ultra-capacitor converter 160 is accompanied by operational loss. Without such a converter, the power converting device 100 may have many benefits.
First, compared to a power converting device having a DC-DC converter according to the related art, a converterless power converting device may improve system efficiency. This is because the loss generated from the ultra-capacitor 160 is absent.
Second, the size of the power converting device 100 may be reduced. The power converting device does not require an inductor for a DC-DC converter, an insulated gate bipolar transistor (IGBT) capable of high-speed switching, and a current transformer (CT) for current measurement. Thus, because the space occupied by the inductor for a converter, the IGBT, and the CT is absent, the size of the power converting device 100 may be reduced.
Third, the cost for manufacturing the power converting device 100 may be reduced. The power converting device 100 may be manufactured as a structure without the inductor for a converter, the IGBT, and the CT.
Accordingly, when a converter is not used and an electric energy storage device such as the ultra-capacitor 105 is used, the power switching device 100 in the related art is advantageous in that loss may be reduced.
However, when the DC link capacitor 150 and the ultra-capacitor 105 are directly connected to each other, a dangerous inrush current may be generated when the ultra-capacitor converter 160 is discharged or the ultra-capacitor 105 must be charged due to a large voltage difference with the DC link. When the current converting device 100 initializes ignition, if one side is completely discharged or the charged voltages are mutually different, this inrush current may be generated when the voltage of the ultra-capacitor 105 is low while the ultra-capacitor 105 is being charged through energy recovery. That is, when the DC-DC converter is simply excluded from a power converting device 100 in the related art, there arises the problem where components may be damaged by an inrush current during initial ignition or charging of the ultra-capacitor 105. Accordingly, the development of a technology for resolving the issue of such an inrush current is urgently required.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.